For about twenty years it has been known to extrude solid thermo-plastic substances. In this case, the thermo-plastic substance generally emerges from the extrusion machine through a die having substantially the shape and the dimensions of the object to be produced, and then passes, still in the plastic state, into a metallic shaping device having the shape and the dimensions of the object to be obtained in a more accurate manner than the die.
When it is sought to apply such a method of operation to expandable thermo-plastic materials, that is to say which contain a blowing agent in their formula, difficulties are met with, due to the fact that at the outlet of the die, the composition expands and thus gives an extruded product, the section of which is not homothetic to that of the die. In fact, in free expansion, the rate of expansion reached by an element of volume dV of the extruded product in its final dimension is a function of the thickness of the die at the level of the element of corresponding original volume dV, and of the distance of this element of original volume dv with respect to the centre of the die. The expansion increases when the thickness referred to increases and decreases when the distance referred to increases.
A die which is to be used in accordance with this technique is therefore very difficult to design.
Furthermore, the considerable friction of the extruded product on the internal surface of the metallic shaping device has the effect of limiting the linear extrusion speed of a section. It is no longer possible to avoid any excess of material at the intake of the shaping device and to eliminate the formation of folds and other irregularities at the surface of the section. The penetration of the extruded product into the metallic shaping device thus takes place against considerable resistance which results in a large stress in tension over the whole of the profile section.
The function of this conventional metallic shaping device is to shape the extruded product and to keep its section constant until it cools at a temperature lower than the softening temperature of the polymer. The time of passage of the extruded product through this shaping device thus constitutes an important factor.
If this time of passage is adequate, the final extruded product possesses a mechanical strength such that it can withstand without deformation the tensile force to which it is subjected. However, this tensile force must generally be high in order to overcome the friction forces developed between the extruded product and the shaping device.
If the extrusion speed is increased, which is undoubtedly a profitability parameter of the operation, the time of passage through the shaping device is reduced and in so doing the intensity of the heat-exchange is diminished, and therefore the mechanical strength of the extruded product is also reduced. Thus, for a shaping device of given length, there exists a maximum traction speed and therefore extrusion speed, beyond which there is a fracture of the extruded product in the shaping device.
In the prior art (see for example German application DOS No. 1,504,310) it has already been suggested to shape the sections by blowing a cooling fluid, this technique making use of the following phenomenon:
At the outlet of a die, the extruded product is permitted to expand freely, and in order to give it the desired shape, blowing is carried out with a cooling fluid at pre-determined points on its surface, towards the end or after the expansion. The cooling fluid thus reaches the material which is already expanded and still in the visco-elastic state, relatively far from the lips of the outlet of the die, and deformation takes place by contraction of the face which is thus cooled. There has thus been obtained by means of a single blowing nozzle, objects having the shape of a half-shell, and by means of several nozzles placed alternately on each side of the extruded sheet or plate, shapes which resemble those of corrugated sheets can be produced. This method does not however enable complex shapes to be produced, and in particular it does not permit the expansion to be controlled.
It is also known to obtain an unfoamed skin on already shaped or partly shaped foamed plastic extrudate by chilling the surface of said extrudate without any shaping action (see for example U.S. Pat. Nos. 3,363,034; 3,317,363; 3,311,681, and 3,299,192).
Moreover U.S. Pat. No. 3,835,209 discloses a process for extruding a foam sheet the thickness of which is controlled by cooling the web immediately after it leaves the orifice lips with a plurality of controllable gas streams. A web of polystyrene foam having a thickness of approximately 0.020 inches can be varied in thickness by at least 0.002 inches using air under pressure varying from 1 to 12 ounces per square inch. However a variation of about 10% in the thickness of a sheet does not permit to foresee the possibility of shaping various and even complex profile sections by a fluid cooling.